Electrolytic capacitors that use conductive polymers as electrolytes are gaining increased shares in the markets due to their excellent impedance characteristics.
The electrolytic capacitors typically use conductive polymers such as solid polypyrrole or polythiophene derivatives as the electrolytes. The conductive polymers allow the electrolytic capacitors that use those conductive polymers to have a remarkably higher electric conductivity (i.e., electron conductivity) than the electrolytic capacitors that use regular liquid as the electrolytes. Therefore, with the capacitors that use the conductive polymers as the electrolytes, it is possible to reduce internal impedance. Thus, such an electrolytic capacitor particularly exhibits excellent characteristics as a high-frequency circuit capacitor. However, with either of the conductive polymer capacitors, the conductive polymers substantially do not have ionic conductivity. Hence, recoverability (i.e. anodization) of dielectric oxide film of the capacitor compare unfavorably with a conventional capacitor that uses an electrolytic solution. As a result, there is a disadvantage with the electrolytic capacitors that it is not possible to form a capacitor with a high withstand voltage. More specifically, usually with an electrolytic capacitor using aluminum as its anode, in a case where for example 40V formation is carried out, a withstand voltage that is actually being used is around 16 V, and with an electrolytic capacitor using tantalum as its anode, in a case where for example 24V formation is carried out, the voltage that is actually being used is around 12 V. Here, the 40 V formation means that a direct voltage applied at a time when a dielectric oxide film is formed on a valve metal surface is 40 V. This ideally obtains a capacitor having a withstand voltage of 40V. In theory, an increase of the formation voltage causes an increase in the withstand voltage in actual use. However in this case, as the formation voltage is made higher, the capacitance decreases, and further even if the formation voltage is increased, the withstand voltage in actual use does not increase in proportion to the formation voltage. In order to solve this problem, the inventors of the present invention already have developed an electrolyte made of an ionic liquid and a conductive polymer (Patent Literatures 1 to 3). The electrolyte was accomplished by finding that an ionic liquid has an excellent anodic oxidation effect of a valve metal, allowing repairing of for example a defect in an aluminum oxide film. By this invention, an electrolytic capacitor with a high withstand voltage was realized.
However, the inventors newly found after further thorough studies on that invention that when a conventional ionic liquid was used, the impedance characteristics easily decrease. In other words, there were cases where the durability of the capacitor was insufficient. Furthermore, the studies by the inventors also resulted in finding that since the conventional ionic liquid has insufficient anodization ability, corrosion and defect of the oxide film is not repairable in time. This ultimately results in breakage of the oxide film, which breakage causes shortage of the electrolytic capacitor. That is to say, it was found that with the electrolyte made of the ionic liquid and conductive polymer, maintaining good durability was one large problem.